FORMATION OF IMAGES. SPHERICAL ABERRATION. 9 



distance of the principal focus, the Image, being formed at an equal 

 distance on the other side of the lens (§ 5), will be of the same 

 dimensions with the Object : whilst, on the other hand, if the 

 object (Fig. 7, a b) be nearer the lens, the image a b will be farther 

 from it, and of larger dimensions ; but if the object A b be farther 

 from the lens, the image a b will be nearer to it, and smaller than 



Fig. 7. 



Formation of Images by Convex Lenses. 



itself. Further, it is to be remarked that the larger the Image 

 in proportion to the Object, the less bright will it be, because the 

 same amount of light has to be spread over a greater surface ; 

 whilst an image that is smaller than the object will be more 

 brilliant in the same proportion. 



9. A knowledge of these general facts will enable the learner 

 to understand the ordinary operation of the Microscope ; but the 



Fig. 8. 



Diagram illustrating Spherical Aberration. 



instrument is subject to certain optical imperfections, the mode of 

 remedying which cannot be comprehended without an acquaint- 

 ance with their nature. One of these imperfections results from 

 the unequal refraction of the rays which pass through Lenses 

 whose curvatures are equal over their whole surfaces. If the course 



